1. Field of the Invention
The present invention relates to GaN-based field effect transistor comprising nitride-based compound semiconductor for use in devices for power electronics and devices for high frequency amplification, and manufacturing process thereof.
2. Description of the Related Art
Since wide band gap semiconductors represented by III-V group nitride-based compound semiconductors exhibit high breakdown filed, good electron transferring characteristics, and good thermal conductivity, they are highly expected as semiconductor devices for high temperature environment, high power, or high frequency. For instance, AlGaN/GaN hetero junction structure involves generation of two-dimensional electron gas at the boundary due to the spontaneous polarization and the Piezo effect. This two-dimensional electron gas has high electron mobility and carrier density, and it is already made available as a device for high frequency. The hetero junction field effect transistor (HFET) employing AlGaN/GaN hetero-structure has low on-resistance and fast switching speed, and operates in high temperature. These characteristics are very suitable in applications for high power switching elements.
Typical AlGaN/GaN HFETs are normally-on type devices in which drain current flows when there is no bias applied to the gate and in which drain current is blocked by applying negative voltage at the gate. In contrast, in high power switching elements, due to securing safety in case the device breaks down, normally-off type devices are preferable in which electric current does not flow when bias (positive voltage) is not applied to the gate and in which electric current flows when positive voltage is applied to the gate.
In order to achieve a normally-off type device, there is a need to adopt MOS structure and studies are conducted in several research organizations (e.g., Non-Patent Document 1). In a structure in the Non-Patent Document 1, n-type layer between a gate and a drain is formed by ion implantation. However, according to the current technology, it is difficult to reduce resistance of n-type layer formed with ion implantation, and it is difficult to accomplish both low resistance and high breakdown voltage.
On the other hand, Patent Document 1 shows a metal-oxide semiconductor field effect transistor (MOSFET) of MOS structure wherein an electron supply layer comprising AlGaN, etc., is etched off at the gate section down to a channel layer and an insulating layer is formed on the etching surface of the channel layer. As regards this structure, space between the gate and the drain is formed of a hetero junction structure comprising AlGaN/GaN, and since the two-dimensional electron gas that is generated due to the hetero junction boundary has high electron mobility, it can avoid increase in on-resistance even if sheet carrier concentration is low, which is needed for maintaining high breakdown voltage. That is, it is a structure suitable for achieving high breakdown voltage with low on-resistance.
[Patent Document 1] International Patent Application WO 03/071607 Publication
[Non-Patent Document 1] Huang W, Khan T, Chow T P: Enhancement-Mode n-Channel GaN MOFETs on p and n-GaN/Sapphire substrates. In: 18th International Symposium on Power Semiconductor Devices and ICs (ISPSD) 2006 (Italy), 10-1.
However, since the AlGaN/GaN hetero junction structure is influenced by a phenomenon called, “current collapse” in which drain current changes with the passage of time with respect to applied voltage, there will be adverse effects upon features of the device such as an increase in on-resistance following application of high voltage between the gate and the drain, an increase in on-resistance at the time of forward flow of current, etc.
As regards the cause of the current collapse, the carrier traps between HFET AlGaN layer and the surface protection film and the carrier traps within semiconductor layer that constitute the element are believed to influence it.